1. Field
One embodiment relates generally to the field of signal demodulation. For example, one embodiment relates to demodulation of signals used in a satellite positioning system.
2. Relevant Background
When an encoded signal is received, the received signal often is severely attenuated due to noise sources and obstructions such as building structures and trees. As a result, it can be difficult to accurately decode the received signal. For example, a received signal from a Global Positioning System (GPS), Galileo and/or other satellite positioning or communication system, each referred to generally herein as a “Satellite Positioning System” (SPS), satellite vehicle can be so severely attenuated that the receiver inaccurately decodes a bit thus causing the received data to be useless. As a result, the receiver has to wait a significant amount of time—e.g., 30 seconds—for the next transmission of the positioning system information before repeating the demodulation function. Thus, the capability of a receiver to accurately demodulate a received signal is often a limiting factor in the performance of a receiver.
One type of demodulation system is known as a differential phase shift key (DPSK) demodulation. Even if the information is not differentially encoded at the transmitter, such as in a SPS system, one can use DPSK at the receiver. This type of demodulation system relies on the value of a previously received bit to determine the value of the present bit. Thus, once an error is introduced—e.g., a bit is incorrectly determined to be a “1” instead of a “0” , the demodulator will produce a series of errors in the subsequent bit values, since the subsequent bit values are dependent upon the previous bit value that was in error. Thus, the resulting demodulated data will be in error. Previously, a demodulator would have had to obtain a re-transmission of the input data and had to repeat the demodulation. As a result, this caused the demodulation process to go more slowly than preferred or resulted in the data not passing at all—thus introducing a delay into the receiver or decreasing sensitivity. In a satellite positioning system, an error in demodulating almanac and ephemeris information constrains the sensitivity and time to first fix (TTFF) of a SPS receiver.